A conventional power conditioner is used to control power flow between a first microgrid and a second microgrid. Each of the first and second microgrids has a neutral terminal. The conventional power conditioner includes a first power converter that is coupled to the first microgrid and that performs AC (alternating current) to DC (direct current) conversion, a second power converter that is coupled to the second microgrid and that performs DC to AC conversion, a DC bus circuit that is coupled to the first and second microgrids, and other circuits. The DC bus circuit includes two capacitors that are coupled to each other. A common node of the capacitors is coupled to the neutral terminals of the first and second microgrids.
When the second microgrid is in an abnormal state (e.g., the second microgrid encounters frequency fluctuation and/or voltage sag), the second power converter transmits active power and/or reactive power from the capacitors to the second microgrid, so as to alleviate power disturbance of the second microgrid. Meanwhile, the first power converter transmits active power from the first microgrid to the capacitors, so that a voltage across the capacitors can be constant.
However, when the second microgrid provides three unbalanced phase voltages, a zero sequence current input received at the common node of the capacitors from the neutral terminal of the second microgrid has a non-zero amplitude, and therefore a voltage across each capacitor has a ripple component, which accelerates aging of the capacitors and may cause malfunction of the conventional power conditioner.
There are two conventional ways to reduce the ripple component of the voltage across each capacitor. The first conventional way is to increase a capacitance of each capacitor. The second conventional way is to include, in the conventional power conditioner, a power supply that provides DC voltages respectively to the capacitors. However, in the first conventional way, the capacitors would be relatively bulky; in the second conventional way, the power supply is required; and in each conventional way, the conventional power conditioner has relatively high manufacturing costs.